1. Field of the Invention
The present invention relates to a novel gear pump.
2. Description of the Related Art
Generally, a conventional fuel supply system of a jet engine (turbo fan engine) used in an aircraft or the like is structured such that a fuel pump (which is also a booster) increases pressure of fuel fed from a fuel tank and then a fuel measuring mechanism determines a flow rate of the fuel, and based on the determination, it supplies some of the fuel to an engine combustor of the jet engine and at the same time returns the remaining fuel or surplus fuel to an inlet of the fuel pump.
In this structure, as the fuel pump, a gear pump 100 shown in FIG. 3 has been heretofore used. In this case, the gear pump is operated by gears within a gear box (AGB: accessory gear box) due to a rotational movement transmitted from the engine. Thus, the discharge rate of the gear pump is generally in proportion to the engine revolutions.
By mean of the gear pump 100, by retaining fuel in closed spaces formed by an inner face of a casing and the gear, it is possible to achieve pressurization of the fuel. In the same figure, IP denotes an inlet port for fuel and EP denotes an exhaust port for fuel.
Recently, a proposal has been made to use a three-gear type gear pump (Double Gear Pump) as a fuel pump instead of using the gear pump 100. The three-gear type gear pump is provided with a driving gear and two driven gears opposed to one another across the driving gear. Fuel is entrapped in closed spaces each formed by two successive (consecutive) gear teeth of each driven gear and a casing whereby the thus retained fuel is pressurized. Therefore, when the driving gear rotates even at a low speed, a sufficient discharge rate can be obtained. See, for example, “GEAR PUMP” fifth edition, Tsuneo Ichikawa (author), (Nikkan Kogyo Shimbun, Ltd. Jan. 30, 1969), and “Investigation and Research on Innovative Aircraft Technological Development No. 1306” The Society of Japanese Aerospace Companies (SJAC), Innovative Aircraft Technological Development Center Mar. 29, 2002 (ISSN 1342-4017).
However, in the three-gear type gear pump as described above, since the driving gear is disposed between the two driven gears, it is subjected to oil pressure at both sides thereof whereby gaps or clearances between the driving gear and the driven gears are generated. As a result, fuel easily leaks from between the driving gear and the driven gears, and therefore, volumetric efficiency is significantly decreased.